In preventive maintenance and mission planning, it is important to calculate the likelihood of failures in a monitored system as symptoms (evidence) are observed. Since many failures frequently have overlapping evidence, it is often the case that ambiguity in fault reasoning will exist when trying to find the root cause failure. The ambiguity can lead to uncertainty in determining a root cause for a failure. For example, the ambiguity can indicate that two system components have failed but there is no way to determine if one or the other or both are the true cause of a fault without the use of complex, and thus expensive, software.